Ferro magnetic core materials and methods of producing same



Feb. 17, 1959 P. ROBINSON FERRO MAGNETIC CORE MATERIALS AND METHODS OF PRODUCING SAME Filed Oct. 15, 1955 FIGJ IN VEN TOR. PRES TON ROBINSON BY f? 55 ATTORNEY United States Patent a FERRO MAGNETICCORE MATERIALS AND METHODS OF PRODUCING SAME Preston Robinson, Williamstown, Mass, assignor to Sprague Electrlc Company, North Adams, Mass, a corporatlon of Massachusetts Application October s, 1955, Serial No. 540,238

6'Claims. or. 29-1825) The present invention relates to magnetic circuit components such as cores and to methods of producing the same. a

Because of variousdesign considerations. known to electrical engineers, magnetic cores for electrical applications should have high permeability and magneticsaturation together with low electrical losses. The so- It is an object of the present invention to produce mag- 2,873,512 Patented Feb. 17, 1959 "ice . taken as corresponding to the fraction retainedupon any one screen of the Tyler series. The term diameter" as used herein is to be taken loosely as the invention is not limited to precise spheres but is applicable to odd shaped pieces of metal.

A feature of the present invention is that the mag? netic ferrite coatings not only electrically insulate the metallic elements from each other, but, by reason of their good magnetic characteristics, do not act as air gaps, that otherwise seriously dilute the metallic particles. In other words, by means of the present invention, the metallic particles are so well insulated from each other that eddy current losses are sharply reduced,

- yet at the same time the assembly of insulated elements Patents Nos.

These cores have very low losses, but benetic circuit components which have the high permeability and saturation of the so-called laminated cores, and which also possess low electrical losses as in the dust cores. Thesecond object of the invention is to develop various procedures for producing-these circuit components. These and further objects of the invention will be apparent from the following description of several of its exemplifications, reference being made to the accompanying drawings wherein:

Fig. 1 is a fragmentary view of a bodying the present invention; and

- Fig. 2 is a plan view of a different form of magnetic circuit component illustrative of the present invention.

-' According to the present invention, individual magnetic metallic elements are provided with an adherent magnetic ferrite coating, and are then assembled into compact form. If desired, the so produced form may be subjected to a sin'teringoperation to 'render it more coherent.

The base materials for the individual coated particles magnetic core emof the present invention preferably are of sof magnetic iron or iron alloys such 'as iron silicon alloys containing up to 6% of silicon, iron-nickel alloys such as that containingby weight 77.2% nickel, 4.8% copper, 1.5%chromium, the balance being iron. It is to be undcrstood that the present invention is not to be limited to is essentially entirely composed of magnetic material.

Suitable magnetic ferrites for use in accordance. with the present invention are the so-called mixed crystal ferrites such as those described in the Snoek et al. U. S. 2,551,711, 2,579,978, 2,636,860, and Crowley U. S. Patent No. 2,575,099. Simple ferrites such as those described in U. S. Patent No. 1,946,964 can also be used.

The adherent ferrite coatings of the present inven tion are formed by providing the surface of the base with a suitable precoating and then heat treating the so precoated material at temperatures of from 900 C. to 1400 C. for periods of from 2 to 5 hours in length. This treatment may be carried out in a number of gases such as technical nitrogen, oxygen or air. For some ferrites such as a manganese ferrite, it is advantageous to employ an oxygen-poor atmosphere in order to hold the stoichiometric proportions of oxygen in the final material so as to correspond to the desired valence of manganese.

When a homogeneous mixed crystal ferrite is used with the present invention, the choice of which mixed crystal system to employ varies with the individual design considerations involved. Frequently, it is desirable to have as high an initial permeability of the ferrite coat: ing as possible. This can beprovided by selecting a composition having a low Curie temperature, but the Curie temperature should not be in the range of temperatures at which the magnetic component is erated.

A very eflfective technique for applying the ferrite coatings is by means of a fluidized bed such as used in the petroleum industry. Because of the nature of the materials employed, it is preferred to use a fixed-bed with this invention but moving beds can also be em ployed. With fixed beds, the velocity of the individual gases employed to suspend the particles is normally chosen so that the bed is in continuous quiescent agitation. With particles of from 40-50 microns in diameter, velocities from 0.01 feet per second to about 0.1 feet per second accomplish this end. With particles in the preferred diameter range noted above, the velocities should be in the order of about 1.5 feet per second. Higher velocities than this may be used, however, but when they are used, slug flow is apt to result and the in dividual particles will not be uniformly treated. When velocities are too low the desired agitation is not ooto be op- "tained. Inasmuch as the conditions for fluidized treat-- ment have been well worked out and are available in the art they will not be further elaborated here. The initial suspension of the metallic particles in a.

fluidized bed may be accomplished with any one of a number of gases, such as hydrogen, oxygen, nitrogen, or air. It is preferred, however, to initially use gases which wilLnot oxidize the individual particles, such as nitrogen, and to utilizethese gases to heat the bed toa temperature of around '500 tof1,O C. Thiscan, 'of course, be accomt'ilishedby the useof gas temperatures of approximate'ly'this same range. Additional heating means, such as jackets,induetion coils, and the like can be employed throughout'the various steps herein specified.

After the bed of particles has been brought to an appropriate temperature, v the externalsurfaces of the individual metal .pieces are oxidized in accordance withthis invention by treatment with an oxidizing gas, such as oxygeuor airfor a period of between IOsecondsan'd five hours depending. upon the .reaction conditionsemploye'd and the final coating thickness desired. It is"b.est..to run several tests to determine the precise degree of oxidation and temperatures for'use with any given type of particle. Certain alloys containsubstituents, such as aluminum, which tend to "'formun'desirable impervious'films upon oxidation. The use of large amounts ofv these film forming metals is, in, general, who avoided.

Occasionally, it is desirable to combine theparticle heating step andthe 'oxidizing'treatment into a single operation in whicheither 'air or oxygen or the like is used to both heat and oxidize the metallic bases employed. Because such a combined operation is difficult to control, it is normallypreferable to avoid it.

After the oxidation in the fluidized'bed has been completed, one or more additional oxides of the type-needed to make the desired ferrite is deposited uponthe individual particles as by the thermal decomposition of an'appropriatc metal alkoxide, as described in the copendingR'obinsonapplication, Serial No. 311,529, filed SeptemberiS, 1-952. Suitable alkoxides are zinc'methoxide, magnesium ethoxide, copper (ous) propoxide, and the like, Other compouentscapable of yielding a metallic oxide besides those listed can, of course, be employed, but the'alkoxides are preferred because of their volatility and ease of decomposition under the reactionconditions involved. Frequently,'it is desirable to aid the passage of these gases through a fluidizedbed by theme of from to 95% of:a non-reactive earrier'gas such as nitrogen.

The individual gases employed during this step of'the process should preferably-be heated toimmediatelybelow their decomposition points in order to facilitate the-production ofthe final coating. Occasionally, it-is necessary to stop the depositionof oxides before the'desiredamount of the'Se-Com onents have been deposited and're-heatthe metallic bases used in order that'the thermal decomposition may proceed-at--a satisfactorily rapid rate.

After the desiredoxide coatings have been deposited, "the particles are heated at from 900 to 1400" C. for a period of one-half to five hours in length as indicated above in orderto. convert the mixed oxides tothe ultimate ferrite.

Since the ferrites as formed above are sometimes adversely affected by rapid-cooling, it is desirable to'slowly cool the hot particles at a rate of about 10*to 100C. per minute. The cooling can be carried out either by a fluidized bed technique or in any other desired manner.

The thickness of the ferrite coatings can vary'widely depending upon the final propertiesdesired. :In general, they should not be so thinthat they are apt 'to'be damaged during subsequent treatment. However, thin I coatings are gen'erally'the'most adherent. A'onernilithickness is about the maximum that .is suitable, .andin general thinnereoatings, from 5 to microns thiclcvfor example, are preferred.

Referring nowto the drawings, Fig. 1 shows in enlarged view one form of magnetic circuit component construction made from base elements in the form of finely .di- 'vided'particles or powder. The base elements are representedat '10, each elementearrying an adherent surface coating 12 of magnetic ferrite. Where the coated particles are "sintered'to'gether, the coatings 'on adjacent particles run together and become integral. Such a particulate construction can, by suitable shaping and molding, be used to make magnetic cores, shells, toroids or the like.

In Fig. 2 there is shown a toroid-type loop core made of strip or .foil 16-spirally'wound. The individual turns of the winding *are insulated from each. other by an adherent magnetic ferrite coating18 which can be applied to one or both'faces of ithe'str ip. JAfter the coated strip is wound up, it can be subjected to .asintering .operation to cause coatings on adjacent faces to cohere together and help bond the turnsin .theidesired shape. Thestrip thickness should .not .be greater than the dimensions indicated above for=theparticles of Fig. 1. In general, however, the strip should be as thin as practical since the eddy current losses for a strip are usually somewhat higher than for the *powder 'type construction. I I

Instead-of having the strip wound'as a continuous spiral, it can be cut into individual lengths which can be laminated together in the usual type of'core construction. Alternatively a continuous spiral-as inFigK-Z can be:cut into twopieces so as to-simplify'the application of windings .to carry the electric currents, when the magnetic component is-used as'part of anfinductor. The cutpieces can ib'e joined together after the windings areapplied. Although only about threeturns-o'f winding 'are shownin Fig. 2,:this is only in the interest of simplicity :and any numberof turns can-beused.

Therfollowing examples of the invention 'are'givenfor purposes of illustration only-and are notto be considered aslimiting theinvention in any manner.

Eicample l A /2" internaldiameter tube-30" high ispl acedbver asupporting manifold'consisting of a gas inlet chamber,

a compact gas distributing sectionand a'20 mesh'supportingscreen. 20 lbs. of silicon-iron alloy particles containing 4% silicon and 96% iron having -an average particle .diameter of about 0.0224" is placed'within the tube above the supporting screen. Heated nitrogen 'at about 1400 C. is .then admitted i'tO the bottom of the column so that itpassesthrough itJat a velocityofiabout 0837 ft. per second. This velocityismaintained for-all gas .flow through the fluidized :treatment. At this velocity .each .individual ,particle is :surrounded by a thin film of gas and is subjected to;arslight:amount ofragitation. After .aperiod of 15;.minutes, technical :oxygen'at 700 C. is passed through the bed'.'for.a;pe12iod tof 'lminute, Manganese ethoxide is then passed :through lthe "bed along with .nitrogenat about 900 ;C; fora period of'10 minutes. At the .end of this time zinc methoxide is passed through the bed along with 80% nitrogen for a period of .15 minutes. The particles are then brought up to the 14.00 C. temperature bypassing heated nitrogen through the bed .as before for a period of 15..minutes. Following this the bed is gradually cooled at a rated 50 C. per.minute by the passage ofqprogressivelycool- 'ing nitrogen through the bed.

Example II Eoatedgparticlesare formed using the same apparatus, reactants, and conditions specified in'Example I except for the following changes:

(1) Nickel propoxide is used instead of manganese ethoxide.

(2 The final-heating isat aj'temperatureof 12-50 "C. for a perio'dof- 4 hours.

lnisome cases it'is possible to'incorporate the metallic ingredients .of either a single .or amixedcrystal "ferrite within the basemateriahand form the product by oxidation without a pre-coatingrstep. ;By'way:of example,"the base can be an alloy ofriron 'and nickel, .or jronnickel andmanganese,.or.it .can.be iron having-thin platings=..of theabovemetals.

#Frequentl the initial iron r iies sns s; 35131633 Ftur b y' decreasing the-nitrogen temperature the rate with the-invention contain impurities suh as carbon and sulfur which haveundesirable efiectsm pon ma magnetic properties-of the final-product. 'I he se'irripurities c ah 'offen be removed to a large extentby initial treatment with hydrogen, as indicated in thefollowing example:

Example IV Coated particles are produced as in Examplel except for the passage of hydrogen at 1000 C. through the bed for a period of 30 minutes immediately before the oxidizing treatment.

The formation of individual circuit elements such as cores from the particles produced in accordance with the preceding discussion in effect follows the broad procedures known to the art. The particles are placed in a mold and subjected to pressures of from 10 lbs. per square inch to approximately 100 tons per sq. inch, removed from them old, and if desired, sintered. On many occasions it is advantageous to employ from 1 to 5% of a binding agent such as gum tragacanth, polyvinyl alcohol,

, dextrin, gum arabic, or the like, to hold the individual particles together during the pressing of the core and immediately thereafter. The use of these binding materials does not have a disadvantageous effect and they can be removed from the core material as by heating to temperatures at which they burn all or are volatilized.

A final sintering of the pressed cores is quite desirable. Not only does it help reduce the air gaps in the pressed material, but if carried out at temperatures of from 900 to 1400 C. and the resulting product is properly cooled as indicated above, the permeability of the ferrite coating usually becomes noticeably higher. Of course, it will be realized that the sintering step serves to more securely bond the individual particles together if carried out for a sufiieient length of time, as for example from 1 to 4 hours.

Example V Particles produced in accordance with the procedure of Example I are placed in a mold one inch in diameter and 6 inches long and subjected to a pressure of 25 tons per square inch. The pressure is then relieved and the pressed mass removed from the mold.

Example VI The core produced in Example V is next sintered at 1200' C. in a nitrogen atmosphere for a period of 3 hours, and cooled in the same atmosphere at a rate of 50 C. per minute until a temperature of 25 C. is reached.

Example VII Particles coated as indicated in Example I are mixed with 2% polyvinyl alcohol in a tumbling drum, and then are treated in accordance with the procedures of Examples V and VI.

When permanent magnetic cores are described, it is advantageous to subject hard ferro-magnetic coated particles to a magnetic field of from 1000 to 10,000 oersteds while cooling them from above the Curie points of all of the substituents involved. This may be quite easily accomplished immediately after the sintering. By this expedient, permanent magnets are obtained which have the advantageous properties of the power magnets such as have been produced by Dean and Davis, U. S. Patent w thin e rpe- Ne. 21391144; and which regsrssrs; i. e; not; eas'llyorrodedoroxidfzediii aii'oxidizingatmos'phere. I

'5 During the cooling of a core sintered as' indicated in tlie .preceditigExample Ia. transverse .field. of. 5,000. oerste'dsiszapplied to the core until a temperatureof 25 C. isreached'.

V Example IX A bizstiiiar siar a;

' ide transformer ,iton

siliconjspnreeled nd passedcontinuofisly filldll gh. oven ill Whlchilil heated gtO 8,00: C. ill. v d

for 3 minutes. The strip is exposed in all treatments so that both faces become coated in each oven. The strip is then passed through a fourth oven where it is heated to 1100 C. for minutes, and then cooled to room temperature at a rate of 10 C. per minute. The resulting material makes a very effective core when merely wound 5 up in spiral form. It is further improved by taking the wound core and reheating it at 1400 C. for 30 minutes in an oxygen-free atmosphere. This reheating causes the coatings to sinter together and the spiral will then be a coherent body that is more readily handled.

30 The above Example IX can be modified by omitting the zinc ethoxide treatment, or by substituting for this treatment a corresponding treatment with manganese methoxide.

Instead of having the iron oxide coating formed by heating in air or oven, it can be formed in other ways as by anodically treating the iron or iron alloy base material in an electrolyte such as ammonia or dilute sodium hydroxide, which does not dissolve iron oxide. The anodic current density should be relatively low, as for example less than 10 amperes per square foot, in order to form the oxide at a relatively slow rate so that it adheres to the anode and does not flake off. The other oxides can also be deposited in other manners, as for example by applying to the base material a finely divided form of oxide such as magnesium oxide suspended in a medium such as a dilute solution of cellulose nitrate or otherresin in an appropriate solvent. Such oxide coatings are, however, more friable and should be subjected to only very gentle handling until after they are fired and converted to ferrite. Such conversion causes the more or less friable oxide layer to become bonded to the iron oxide layer underneath it.

The ferrite coatings of the present invention can also be deposited on non-magnetic bases. For example, magnesium-manganese ferrites such as those containing manganese in amounts corresponding to about 15% M1130 make good rectangular hysteresis bodies.

These ferrites can be deposited in a layer up to abou 5 mils thick on ceramic forms such as rings having an internal diameter of 50 mils, an external diameter of 70 mils and 30 mils thick. All the ferrite ingredients can be deposited by thermal deposition or by oxidation of metal coatings or partly one way and partly another. In general, the ferrites should have approximately stoichiometric composition.

As many apparently widely different embodiments of this invention may be made without-departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments set forth except as defined in the appended claims.

This application is a continuation-in-part of application Serial No. 310,719, filed September 20, 1952.

What is claimed is:

1. A magnetic circuit component comprising an assembly of thin ferromagnetic metal individual finely divided particles less than about 40 mils thick, the elements gem-512 7 being electrically insulated from? each other; hy sur fa'ce coatings less than 1 mil thick of ;magnetic ferrit'esg;

2. The combination of claim ,1 in which the coated particles are sintered together, the coatings on adjacent particles'being integral. 3. A ferro-magnetic electrical core consisting of individual magnetic iron finely divided particles less than about 40 mils thick provided with magnetic ferrite coatings less than 1 mil thick.

4. A process of preparing a magnetic circuit component, which process includes the steps of providing ferro-magnetic metal individual finely divided particles less than 40 mils thick with surface coatings of magnetic ferrites less than 1 mil thick, and sintering and running together coatings on adjacent particles to form an integral particle coating and hold the particles together in an assemblyof such elements to form a coherent body.

5. The combination of claim 1 in which the thin metal elements are between 4 and 25 microns in thickness, and

their surface coatings are between 5 and 15 microns gthick.

2,068,658 Cox Jan. 26, 1937 2,179,810 Brill Nov. 14, 1939 2,365,720 Neighbors Dec. 26, 1944 2,418,467 Ellis et al. Apr. 8, 1947 2,563,520 Faus Aug.7, 1951 2,783,207

Tombs Feb. 26, 1957 

